1. Field of the Invention
This invention relates to x-ray scanners and more particularly to a method for calibrating devices of this type which utilizes the image of one or a number of calibration standards. The invention is also concerned with a system for carrying out said method.
2. Description of the Prior Art
In order to examine a patient, it is becoming an increasingly common practice to employ x-ray devices known as scanners which produce images of cross-sections of the patient's body. These devices are based on the physical phenomenon of absorption of x-rays by the human body. This absorption is directly related to the distance x of travel of the x-rays within the body in accordance with the formula: EQU I=I.sub.o e.sup.-bx
where:
I.sub.o is the intensity of the radiation which enters the human body,
I is the intensity of radiation which passes out of the human body,
b is a coefficient of attenuation which depends on the body through which the radiation passes.
In a logarithmic measurement scale, the attenuation I/I.sub.o is equal to bx or in other words proportional to the distance x.
As shown in FIG. 1, a scanner is essentially constituted by an x-ray source 10 associated with a detection device 11, these two elements being disposed in a fixed geometrical relationship to each other in order that the body to be examined may be positioned between them. Furthermore, they are supported by a structure (not shown) which is capable of rotating about the body to be examined so as to irradiate the body at different angles. The x-ray source, which is controlled by a device 13, emits x-rays in an angular sector which has a sufficient width to illuminate the entire cross-section of the body. The detection device 11 has the shape of an annular sector, the length of which is adapted to the width of the x-ray beam and is constituted by a large number of elementary detectors 12 in juxtaposed relation to each other.
In order to obtain an image of the cross-section of the human body through which the x-ray beam passes, the structure which supports the source 10 and the detection device 11 is caused to rotate about the body and output signals of the elementary detectors 12 are measured and then suitably processed in accordance with known methods in order to obtain a representative image of the cross-section. For this processing operation, the elementary detectors 12 (also known as channels) are connected to an electronic device 14 which carries out in the first place a computation of the logarithm of the signals received so as to obtain a signal whose amplitude is proportional to the attenuation of the x-rays.
As a result of different phenomena which will not be explained here, the amplitude of said signal in the case of each elementary detector or channel is not proportional to the attenuation which has in fact been sustained. In consequence, in order to overcome this disadvantage, various methods have been devised which consist for example in recording the output signals of the channels in the presence of bodies having known dimensions and a known coefficient of absorption so as to compute the attenuations (calculations of logarithms) and to compare these measured attenuations with values computed as a function of the dimensions and of the absorption coefficient of the body or calibration standard. These comparisons make it possible to deduce a law of correspondence or a modifying law between the measured values and the values which should be obtained. This law can be in the form of correspondence files or of mathematical formulae representing this correspondence in respect of each detection channel.
The standards employed for carrying out these so-called calibration measurements are for example blocks of different thicknesses which are introduced in proximity to the x-ray source, thus entailing the need for handling operations at the level of the source in order to introduce and withdraw these blocks. Furthermore, the shapes of said blocks and their positions are remote from the shape and position of the body of the patient to be examined, thus increasing the nonlinearity of the system.
In U.S. Pat. No. 4,352,020, it is proposed to use circular blocks 15, 16 and 17 having different diameters which are placed at the center of rotation of the support structure. This makes it possible to come closer to the conditions of measurements to be performed on the body to be examined. This patent also proposes to make use of a cone-shaped standard having a circular cross-section which is displaced transversely with respect to the beam so as to obtain different attenuation lengths. With the standards described, the measurements are performed with respect to a predetermined position of the support structure and in the case of each standard.
FIG. 2 shows the shape of three response curves 20, 21 and 22 of the attenuation as a function of the position of the channels in the case of measurements on three standards of circular shape. The measured values are represented by the dots and vary about a mean value which represents the theoretical value in a linear system. These curves can be employed as follows: when the measured signal corresponds to a point A, it will be deduced from this that the linear signal is the point A' of the mean curve 20. When the measured signal corresponds to a point B located between the curves 20 and 21, the linear signal will be deduced therefrom by interpolation between the curves 20 and 21. This interpolation can be computed in accordance with a linear law or more generally a polynomial law.
The curves 23 and 24 of FIG. 3 show in another form the principle of calibration at the level of a channel. These curves describe, in a given channel, the attenuation as a function of the thickness x of measured values (curve 23) and in respect of computed values (straight line 24). In fact, the measured values produce points which are connected to each other in accordance with a predetermined law which may be linear or polynomial so as to obtain a continuous curve. When an attenuation is measured, this corresponds for example to point C of curve 23 and the linear value corresponding to the point C' of curve 24 is deduced therefrom.
The U.S. patent cited earlier describes an apparatus in which the correspondence between the measured values and the real values of attenuation is established by means of a system of files created during the calibration operation. In regard to interpolation, the patent proposes linear, cubic and biquadratic interpolations, but the linear interpolation is alone described in detail.
The methods of calibration which have been briefly described in the foregoing are distinguished by the fact that they carry out a processing operation on the measured signals in order to obtain an image which is free from artifacts or in other words without flaws. However, it is not always easy to obtain an image of this quality, especially on account of the fact that the calculations are made with certain approximations which can be very rough in some instances.
In consequence, one object of the present invention is to carry out a method which processes, not the measured signals themselves, but the image obtained so as to eliminate circular artifacts.
Another object of the present invention is to provide a system which makes it possible to carry out said method.